The present invention relates to molded thermoplastic articles, and more specifically, to injection-molded thermoplastic foam articles.
Conventional injection molding typically involves injecting a molten thermoplastic at a high pressure into the cavity of a closed mold. The injection pressure can range anywhere from between 2,000 psi to 10,000 psi, depending on the article being made. After the thermoplastic has sufficiently cooled to a hardened condition, the mold is opened and the molded article is removed.
In conventional molding processes, a problem that is often encountered is the formation of “sink marks” or distortions on the surface of the molded article opposite a projection, such as a rib, boss, bracket or return. A sink mark is a visually perceptible divot or indentation in the surface of the article that is largely created as a result of uneven material shrinkage during the curing segment of the injection molding process.
The formation of sink marks can be caused by a variety of factors, such as, use of insufficient injection and pack out pressures during the molding process. One of the largest contributing factors to the formation of sink marks on the surface of molded articles is the ratio of the thickness of the projection to the thickness of the structure or wall stock from which the projection extends. It is well known that when conventional injection molding is used to manufacture a molded thermoplastic article that a sink mark will form in the surface of the article when a thermoplastic olefin is used and the thickness of the projection is greater than about 25% of the thickness of the structure. Furthermore, when other certain thermoplastic materials are used, such as for example crystalline, semi-crystalline or amorphous materials, a sink mark will form in the surface of the article when the projection is 40% or greater than the thickness of the structure. While a projection having a thickness greater than 25% of the thickness of the structure is not always required, there are many instances when a projection greater than the above ratio is highly desirable and beneficial. For example, a larger projection can provide greater structural integrity to the article and can provide a larger attachment area for attaching the molded article to other objects.
FIGS. 1-3 illustrate three different examples of conventional injection molding configurations. Referring to FIG. 1, the molded article 10a includes a front surface 12a and a rear surface 14a. Front surface 12a is a Class “A” surface. As used herein a Class “A” surface is intended to refer to a surface that is visually decorative and smooth to the unaided eye or, in other words, is visually free of unintentional distortions, such as “sink marks,” dimples, indents, divots or the like. Such Class “A” surfaces include non-textured and purposely textured surfaces, such as haircelled surfaces and the like, that are visually free of unintentional distortions.
Molded article 10a also includes a structure or wall stock 16a and a projection 18a. The structure 16a includes a front wall 20a which is defined by a portion of front surface 12a and a rear wall 22a defined by a portion of rear surface 14a. Projection 18a includes a first sidewall 24a and a second sidewall 26a, both of which are defined by portions of rear surface 14a. Projection 18a also includes an end wall 28a defined by a portion of rear surface 14a. Furthermore, the molded article 10a also includes a joint 30a between the structure 16a and projection 18a. The joint 30a also is defined by portions of rear surface 14a and is blended with sidewalls 24a and 26a of projection 18a and rear wall 22a of structure 16a. 
Sidewall 24a and sidewall 26a of projection 18a are separated by distance Tp, which defines the thickness of projection 18a. Additionally, front wall 20a of structure 16a and rear wall 22a of structure 16a are separated by distance Ts, which defines the thickness of structure 16a. It is well known in the art that to provide a Class “A” surface using thermoplastic olefins (TPO), the distance Tp must be 25% or less than the distance Ts and the joint 30 must be at about a 90 degrees angle. If using certain other thermoplastic materials, such as those mentioned above, the distance Tp must be 40% or less than the distance Ts.
Comparing FIGS. 2 and 3 to FIG. 1, it is also generally well known in the standard injection molding art that when joint 30b between structure 16a and projection 18b is curved (as shown in FIG. 2) or the thickness Tp of projection 18c is greater than 25% to 40% (depending on the material) of the thickness Ts of structure 16c (as shown if FIG. 3), a visible distortion, such as sink mark 32, will form in front surfaces 12b (FIG. 2) and 12c (FIG. 3) of the structure during the injection molding process. It is believed that distortions occur because the relatively large mass of thermoplastic material located in the area of the wall joint, as compared to the rest of the structure, cools at a relatively slower rate as compared to the thinner portions of the structure. As the greater concentration of heat in the larger mass continues to cool to ambient, material shrinkage continues longer in the larger mass sections than in the surrounding thinner sections. The resulting delay in arriving at the ambient temperature allows the front surface to be drawn longer and further inward thereby causing a surface distortion, such as a sink mark.
While surfaces with unintentional distortions may be tolerated for non-visible, lower end products, when the products are required to have a high quality appearance, such as a Class “A” surface in exterior and interior automobile parts, unintentional distortions are highly undesirable and often make the molded article unsuitable for use. It is desirable to have Class “A” surfaces on auto parts, such as bumpers, sun-visors, cab extenders, side fairings, hoods or fenders.
Therefore, there remains a need for molded articles having relatively large projections and a Class “A” surface on the opposite side thereof, which surface is visibly free of sink marks or distortions.